1. Field of the Invention
The present invention relates to an adaptive predistortion method and device for power amplifiers dedicated in particular but not exclusively to spectrally efficient microwave mobile communication equipments.
2. Brief Description of the Prior Art
With the increasing demand on the RF and microwave spectrum, caused by the proliferation of wireless communications and satellite networks, more spectrally efficient modulation techniques will have to be developed. Linear modulation methods, like M-ary QAM, meet this requirement with high units of bits per second per Hertz. But since it has a high envelope variation, their performance is strongly dependent on the linearity of the transmission system. In addition, modern wireless radio systems like mobile cellular and emerging Personal Communication Systems (PCS) require a high power efficiency to extend the battery life of the portables. To maximize the power added efficiency and the power output, the power amplifier is often operated near saturation where the input/output power characteristics become nonlinear. Unfortunately, if linear modulation with fluctuating envelope is used in conjunction with a highly efficient nonlinear amplification, distortion and spectral spreading into adjacent channels will occur. In order to reduce these undesired effects and meet the desired power and spectral efficiency, linearization techniques have been introduced.
A variety of linearization methods have been reported and many different ways can be used to segment this topic. Factors such as average transmitter power, operating bandwidth, power efficiency, adaptability and complexity are significant considerations in design compromises that can be used to categorize the different techniques. In general, all these techniques are, by any measure, derived from three main types named:
i) Feed-forward (R. Meyer, R. Eschenbach and W. Edgerley, Jr. "A wide-Band Feedforward Amplifier", IEEE J. of Solid-State Circuits, vol. sc-9, no. 6, pp. 442-428, December 1974), which includes an open loop configuration, can handle a multicarrier signal but can not easily be controlled against the effects of drift. Moreover, their low power efficiency make it suitable in base station only. A good analysis of adaptation behavior has been presented in J. Cavers, "Adaptation Behavior of a Feedforward Amplifier Linearizer", IEEE Transactions on Vehicular Technology, vol. 44, no. 1, pp. 31-40, February 1995; PA1 ii) Feedback (A. Bateman & D. Haines, "Direct Conversion Transceiver Design for Compact Low-Cost Portable Mobile Radio Terminals" IEEE Conf. pp. 57-58, 1989), which presents an excellent reduction of out-of-band emissions, is relatively easy to implement. However, stability requirement limits its bandwidth because of its critical dependence on the loop delay; and PA1 iii) Predistortion (N. Imai, T. Nojima and T. Murase, "Novel Linearizer Using Balanced Circulators and Its Application to Multilevel Digital Radio Systems", IEEE Transactions on Microwave Theory and Techniques, vol. 37, no. 8, pp. 1237-1243, August 1989), this technique has historically been the most common method in analog implementation. This method uses a nonlinear element which precedes the device to be compensated, its gain-expansion characteristic cancels the gain compression of the amplifier. Like feed-forward, it has an open loop configuration and therefore is very sensitive to drifts. PA1 the signal to be transmitted is a digital signal; PA1 the predistortion amplitude and phase look-up table means are digital predistortion amplitude and phase look-up table means for producing a digital predistorted signal; PA1 the adaptive signal predistorting device comprises a digital-to-analog converter for converting the digital predistorted signal into an analog predistorted signal and a quadrature modulator for converting the analog predistorted signal into a microwave signal supplied to the input of the power amplifier; and PA1 the means for producing a second feedback signal comprises a microwave coupler for supplying a portion of the amplified output signal of the power amplifier to a quadrature demodulator producing, in response to this portion of the amplified output signal of the power amplifier, a demodulated analog signal and an analog-to-digital converter for converting the demodulated analog signal into the second feedback signal, in digital form. PA1 the one-dimensional predistortion amplitude look-up table for producing a predistorted amplitude in response to the amplitude component of the signal to be transmitted; PA1 the one-dimensional predistortion phase look-up table for producing a predistorted phase in response to the predistorted amplitude; and PA1 means for combining the phase component of the signal to be transmitted, the predistorted amplitude and the predistorted phase into the predistorted signal at the output of the predistorter means.
In recent years, the technology progress of Digital Signal Processors (DSP) has been one of the motive of the imminent course toward digital modulation techniques. Actually, the digital signal can be processed in such a way that greater bandwidth efficiency and voice quality can be obtained. In addition various applications such as generation of accurate gain and phase matching in two quadrature modulating signals, real-time compensation for channel impairments and the benefits of fast computational machines have motivated the use of these processors in several methods of linearization. These techniques are called Digital Linearization Techniques.
One of the features of the digital techniques is the control against the effects of drift. It is well known, that the power amplifier characteristic are quite sensitive to temperature variation and some unbalance in the linearization process can be occurred. In order to overcome this problem and avoid the effect of device power supply precision and drifts produced by switching between channels, adaptability is needed. In this way, an adaptive digital predistorter is the most promising technique that can be applied to narrow band Personal Communication Service using a DSP. The first successful work was presented by Y. Nagata, "Linear Amplification Technique for Digital Mobile Communications", in Proc. IEEE Veh. Technol. Conf. Sans Francisco, Calif., pp. 159-164, 1989 using a two-dimensional Look-Up Table (LUT) technique with adaptive digital feedback at baseband and pulse shaping filter prior to predistortion. This technique has shown the advantage that any order of nonlinearity and any modulation format can be supported. Followed later by J. Cavers, "Amplifier Linearization Using a Digital Predistorter with Fast Adaptation and Low Memory Requirement", IEEE Transactions on Vehicular Technology, vol. 39, no. 4, pp. 374-382, November 1990 and M. Faulkner, T. Mattsson and W. Yates, "Adaptive Linearization Using Predistortion", in Proc. 40th IEEE Veh. Technol. Conf. pp. 35-40, 1990, several drawbacks were eliminated using a one-dimensional table. This has made possible that less memory is needed and therefore, the convergence time has been reduced. These previous techniques were based on iterative algorithms.
An interesting idea was proposed by T. Wilkinson, "An Assessment of the Performance of Linearization Schemes in the Australian Mobilsat System by Simulation", IEE 6th Int. Conf. on Mobile Radio, London, pp. 74-76, 1991 using two look-up tables, one for the amplitude and the second for the phase. Each LUT includes one hundred entries covering the range of input levels and linear interpolation is used to determine values between entries. This later technique does not consider any adaptability dedicated to drift correction.